Abstract:

A method of high-energy particle imaging by individual particle counting
with an active pixel direct bombardment detector. The method includes the
step of providing an active pixel direct bombardment detector including
an array of pixels. Each pixel is characterized by a reset time constant.
The method further includes sampling the pixel voltage of each pixel at a
first time. The method further includes applying a pixel reset voltage to
each pixel for a reset time interval less than the reset time constant.
The method further includes sampling the pixel voltage of each pixel at a
second time. The method further includes computing a difference between
the sampled pixel voltages at the first and second times. The sampling
and the applying of the reset voltage may be periodic. A direct
bombardment detector is also provided.

Claims:

1. A method of high-energy particle imaging by particle counting with an
active pixel direct bombardment detector, the method comprising the steps
of:providing an active pixel direct bombardment detector including an
array of pixels, each pixel characterized by a reset time
constant;sampling the pixel voltage of each pixel at a first
time;applying a pixel reset voltage to each pixel for a reset time
interval less than the reset time constant;sampling the pixel voltage of
each pixel at a second time, andcomputing a difference between the
sampled pixel voltages at the first and second times to detect a particle
interaction event occurring between the first and second times.

2. The method of claim 1 wherein the array of pixels is a 2D array.

3. The method of claim 1 wherein the direct bombardment detector is an
electron detector.

4. The method of claim 3 wherein the electron detector includes a sensing
volume with a dimension parallel to an incident radiation beam less than
50 microns.

5. The method of claim 4 wherein the sensing volume has a dimension
parallel to an incident radiation beam less than 15 microns.

6. The method of claim 1 wherein the applying of the pixel reset voltage
includes sending a reset pulse to control a reset switch connected to a
reset voltage to selectively connect each pixel to the reset voltage.

7. The method of claim 6 wherein the reset time constant is approximately
a time interval necessary for a voltage response of each pixel to the
reset voltage to reach approximately 63% of its final asymptotic value.

8. The method of claim 1 wherein the computing of the difference includes
comparing analog signals of the sampled pixel voltages.

9. The method of claim 1 wherein the computing of the difference includes
comparing digital signals of the sampled pixel voltages.

10. A method of high-energy particle imaging by particle counting with an
active pixel direct bombardment detector, the method comprising the steps
of:providing an active pixel direct bombardment detector including an
array of pixels, each pixel characterized by a reset time
constant;sampling the pixel voltage of each pixel at a first
time;applying a pixel reset voltage to each pixel for a reset time
interval less than the reset time constant;sampling the pixel voltage of
each pixel at a second time, anddetecting a particle interaction with a
given pixel using a difference between the sampled pixel voltages at the
first and second times for such given pixel.

11. A method of high-energy particle imaging by particle counting with an
active pixel direct bombardment detector, the method comprising the steps
of:providing an active pixel direct bombardment detector including an
array of pixels, each pixel characterized by a reset time
constant;periodically applying a pixel reset voltage to each pixel for a
reset time interval less than the reset time constant;periodically
sampling the pixel voltage of each pixel subsequent to each periodic
application of the pixel reset voltage to each pixel;computing a
difference between each sampled pixel voltage and each immediately
subsequent sampled pixel voltage to detect a particle interaction event
occurring between the sampling of compared pixel voltages.

12. A direct bombardment detector for high-energy particle imaging for use
with a radiation beam, the detector comprising:an array of pixels, each
pixel characterized by a reset time constant; the array of pixels being
positionable with the radiation beam being incident upon the array of
pixels;a reset voltage;a reset switch connected to the reset voltage and
each pixel, the reset switch being configured to selectively connect each
pixel to the reset voltage for a reset time interval less than the reset
time constant in response to the a reset pulse;a sampling device
configured to sample the pixel voltage of each pixel at a first time
prior to connection of each pixel to the reset voltage and at a second
time after connection of each pixel to the reset voltage; anda control
device configured to compute a difference between the sampled pixel
voltages at the first and second times.

13. The detector of claim 11 wherein the direct bombardment detector is an
electron detector.

14. The detector of claim 12 further includes a sensing volume with a
dimension parallel to the incident radiation beam less than 50 microns.

15. The detector of claim 14 wherein the sensing volume has a dimension
parallel to an incident radiation beam less than 15 microns.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002]Not Applicable

BACKGROUND

[0003]The present invention relates in general to a high-energy particle
imaging, and more particularly, to a method of high-energy particle
imaging by computing a difference between sampled pixel voltages.

[0004]Conventionally, either photographic emulsions or electronic image
sensor based cameras using down-converting scintillator screens are used
in a transmission electron microscope (TEM). The scintillator (or
phosphor) screen converts the impinging electron image into a visible
light image that can be recorded with the photon sensitive devices. These
existing detection techniques have several drawbacks, such as limited
sensitivity, limited resolution, poor usability, and time inefficiency.

[0005]Photographic film has been a long-standing standard for electron
imaging due to the very high modulation transfer and a large field of
view that can be provided. The cumbersome post-acquisition steps
associated with film have lead to near complete replacement of the
technique, however, by electronic recording methods.

[0006]Charged coupled device detectors (CCDs) are now widely used in
electron microscopy. These detectors overcome the time-consuming steps of
loading, unloading, processing, and digitizing film by providing a
digital output directly. Commonly available CCD detectors have formats up
to 4096 by 4096 pixels (4K×4K), although they fall short of
delivering the full resolution anticipated by the pixel count alone. CCD
detectors require the use of a fluorescent scintillation screen to
convert the electron image to a photon image. Unfortunately, with each
primary electron event, the size of a fluorescent spot produced within
the scintillation screen is much larger than the CCD pixel size. Although
scintillator material layer thickness can be reduced to minimize the spot
size, sensitivity is sacrificed as the number of photons produced per
incident electron is also reduced. For example, in electron microscopy at
300 KeV, the full width at half maximum of the spot from a typical
scintillator material is about 30 μm. However, the full width at 1% is
200 μm. The large spread of light reduces the effective resolution of
the CCD camera, which often has pixels on the order of 15 μm, by at
least a factor of two in each dimension, thus rendering the effective
resolution of a 4K CCD camera to 2K×2K. This is far less than the
resolution achieved by film, which for a 8 by 10 cm sheet is on the order
of 8K×10K.8K. Nevertheless, except in rare instances, CCDs have
replaced film because of other shortcomings associated with film.

[0007]A new type of detector for TEM that overcomes the limitations of
scintillators and may deliver, or even exceed, the full resolution of
film is disclosed in U.S. Pat. No. 7,262,411 entitled "Direct Collection
Transmission Electron Microscopy" the complete contents of which are
incorporated herein by reference. A detector based on active pixel
sensors is used in direct bombardment mode to achieve direct detection of
primary electrons without use of a scintillator screen. This new detector
includes a pixel array comprising charge collection diodes that collect
secondary electrons generated when a primary electron passes through the
thin epitaxial silicon layer in which the p-n junction of the diode is
formed. These detectors may achieve relatively high-speed readout, high
spatial resolution, and very high sensitivity to single primary
electrons.

[0008]A drawback of this and many other detectors currently used for
electron imaging is that they intrinsically record quantities of energy
deposited by the incident particles as they traverse a thin sensing
volume in the detector. These thin-section detectors, which happen also
to include photographic film and scintillator screens, collect a small
fraction of the primary particle energy as it scatters inelastically
(losing some energy as it scatters) while passing through the sensing
volume.

[0009]By contrast, some detectors used in x-ray and high-energy particle
detection, rely on completely stopping the particle and collecting all of
its energy, by using relatively thick sensing volumes. While these thick
detectors can accurately (with relatively low noise) record the energy of
the incident particle, they suffer from poor spatial resolution due to
the lateral scattering the incident particle can undergo in the sensing
volume, and do not prove to be very good imaging detectors. Reducing the
sensing volume thickness of the detector improves spatial resolution, but
results in lower collected signals in the thin-section detectors.
Additionally, the signal in the thin-section detectors is highly
variable, due to the fact that the energy deposited by the incident
particle in the thin sensing volume varies statistically according to the
Landau distribution, which has a long tail extending to high energy (up
to the total energy of the incident electron). This varying nature of the
deposited energy in the thin-section detectors introduces an additional
SNR penalty that is particularly apparent under low dose imaging
conditions.

[0010]Counting methods have been proposed for use with both bulk detectors
(also known as hybrid detectors) and thin-section detectors such as CCDs,
active pixel sensors, and strip detectors. Counting avoids the noise
associated with integrating methods by simply recording either the
presence or absence of an incident electron in a pixel. To be successful
however, counting requires very high frame rates if practical electron
beam intensities are to be used. Counting methods also afford the
opportunity to improve spatial resolution by performing analysis of the
cluster of pixels that receive charge from an incident electron.

[0011]Conventional read-out methods used with active pixel sensors are
considered too slow, however, for large arrays to be used with electron
beams of practical intensities. This limitation arises because of the
need to clear charge from the array between exposure frames.

[0012]Any pixel in an active pixel imaging-sensor is able to detect input
signals (from light, electrons, etc.) by integrating ionization electrons
in its sensing volume and collecting the total charges in its photodiode.
During the pixel reset, the photodiode of each pixel is connected to a
reset voltage and the charge in the photodiode is cleared out. At the
completion of the reset, the photodiode enters an integration mode and
starts to collect ionization electrons that are generated from an
external source, such as light or electron illumination.

[0013]Referring now to FIG. 1 there is depicted a symbolic illustration of
a photodiode voltage response profile of a single pixel to a constant
external signal input (constant illumination) and a reset pulse profile
(with photodiode voltage represented along the vertical axis and time
represented along the horizontal axis). The external signal drives the
diode voltage down linearly with increasing integration time. When the
pixel is reset again, the voltage recovers (exponentially) and returns
back to the reset level. It is noted that the voltage may actually be
driven up (depending upon the particular pixel design setup utilized).

[0014]Referring now to FIG. 1, a prior art method to read such a sensor is
to sample the voltage output of the photodiode (also referred to as
photodiode voltage and pixel voltage) at the end of each integration
period such as at time points as indicated at time instances 1' and 2'.
Another prior art readout method is to sample the voltage both at the
beginning and end of each integration period (time instances 1, 1', and
2, 2'). The latter readout method is often called Correlated Double
Sampling (CDS), because for each pixel reset the photodiode voltage is
sampled twice and only the difference between the two sampled voltages is
used for the final image. The CDS method effectively eliminates the reset
noise (kTc noise), thus is a preferred method for low-noise applications.

[0015]In the electron counting case, a single electron strikes a pixel and
deposits a quantity of charge almost instantaneously. Referring now to
FIG. 2 there is depicted a symbolic illustration of a photodiode voltage
response profile resulting when an incident electron hit the pixel and a
reset pulse profile (with photodiode voltage represented along the
vertical axis and time represented along the horizontal axis). FIG. 2
shows the photodiode voltage profile when an incident electron hits the
pixel in the middle of the integration time period after the first reset
pulse. The photodiode voltage drops down to the low level after the
electron hit (as indicated at a time instance in the middle of the
integration time interval between time instances 1 and 1'), and it only
begins to recover when the next pixel reset is asserted Oust after the
time instance 1').

[0016]Referring now to FIG. 3 there is depicted a sample plot of voltage
values of a video output recorded from a single pixel using the CDS
method as plotted over the course of 50 frames. A high-energy electron
event is easily observed in frame 22. The CDS method requires two reads
for each frame and a relatively long reset period, to ensure accurate
quantification of the signal charge collected.

[0017]As illustrated by both FIG. 1 and FIG. 2, the voltage recovery
process that occurs during the assertion of the pixel reset follows an
exponential curve characterized by a reset time constant. The reset time
constant is specific to the device design and is associated with the
reset voltage, resistance, and capacitance of the circuitry. The reset
time constant can become particularly problematic in large arrays and in
some cases can require reset periods as long 10 to 30 microseconds. In
high frame rate applications, such as electron counting for TEM, this
time constant can become an important bottleneck to attaining high frame
rates.

[0018]Accordingly, there exists a need in the art for an improved
high-energy particle imaging used with an active pixel direct bombardment
detector in comparison to the prior art.

BRIEF SUMMARY

[0019]According to an aspect of the present invention, there is provided a
method of high-energy particle imaging by particle counting with an
active pixel direct bombardment detector. The method includes the step of
providing an active pixel direct bombardment detector including an array
of pixels. Each pixel is characterized by a reset time constant. The
method further includes sampling the pixel voltage of each pixel at a
first time. The method further includes applying a pixel reset voltage to
each pixel for a reset time interval less than the reset time constant.
The method further includes sampling the pixel voltage of each pixel at a
second time. The method further includes computing a difference between
the sampled pixel voltages at the first and second times to detect a
particle interaction event occurring between the first and second times.

[0020]According to various embodiments, the array of pixels is a 2D array.
The direct bombardment detector may be an electron detector. The electron
detector may include a sensing volume with a dimension parallel to an
incident radiation beam less than 50 microns, or even less than 15
microns. The applying of the pixel reset voltage may include sending a
reset pulse to control a reset switch connected to a reset voltage to
selectively connect each pixel to the reset voltage. The reset time
constant may be approximately a time interval necessary for each pixel,
in response to the reset voltage, to reach approx 63% of its final
asymptotic voltage across the pixel. The computing of the difference may
include comparing analog or digital signals of the sampled pixel
voltages.

[0021]According to an aspect of the present invention, there is provided a
method of high-energy particle imaging by particle counting with an
active pixel direct bombardment detector. The method includes the step of
providing an active pixel direct bombardment detector including an array
of pixels. Each pixel is characterized by a reset time constant. The
method further includes sampling the pixel voltage of each pixel at a
first time. The method further includes applying a pixel reset voltage to
each pixel for a reset time interval less than the reset time constant.
The method further includes sampling the pixel voltage of each pixel at a
second time. The method further includes detecting a particle interaction
with a given pixel using a difference between the sampled pixel voltages
at the first and second times for such given pixel.

[0022]According to another aspect of the present invention, there is
provided a method of high-energy particle imaging by particle counting
with an active pixel direct bombardment detector. The method includes
providing an active pixel direct bombardment detector including an array
of pixels. Each pixel is characterized by a reset time constant. The
method further includes periodically applying a pixel reset voltage to
each pixel for a reset time interval less than the reset time constant.
The method further includes periodically sampling the pixel voltage of
each pixel subsequent to each periodic application of the pixel reset
voltage to each pixel. The method further includes computing a difference
between each sampled pixel voltage and each immediately subsequent
sampled pixel voltage to detect a particle interaction event occurring
between the sampling of compared pixel voltages.

[0023]According to yet another aspect of the invention there is provided a
direct bombardment detector for high-energy particle imaging for use with
a radiation beam. The detector includes an array of pixels. Each pixel
characterized by a reset time constant. The array is positionable with
the radiation beam being incident upon the array of pixels. The detector
further includes a reset voltage. The detector further includes a reset
switch connected to the reset voltage and each pixel, the reset switch
being configured to selectively connect each pixel to the reset voltage
for a reset time interval less than the reset time constant in response
to the a reset pulse The detector further includes a sampling device
configured to sample the pixel voltage of each pixel at a first time
prior to connection of each pixel to the reset voltage and at a second
time after connection of each pixel to the reset voltage. The detector
further includes a control device configured to compute a difference
between the sampled pixel voltages at the first and second times.
Further, the detector may be an electron detector. The detector may
further include a sensing volume with a dimension parallel to the
incident radiation beam less than 50 microns, or even less than 15
microns.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]These and other features and advantages of the various embodiments
disclosed herein will be better understood with respect to the following
description and drawings, in which like numbers refer to like parts
throughout, and in which:

[0025]FIG. 1 is a symbolic illustration of a photodiode voltage response
profile of a single pixel to a constant external signal input (constant
illumination) and a reset pulse profile (with photodiode voltage
represented along the vertical axis and time represented along the
horizontal axis);

[0026]FIG. 2 is a symbolic of a photodiode voltage response profile of a
single pixel when an incident electron hits the pixel, and a reset pulse
profile (with photodiode voltage represented along the vertical axis and
time represented along the horizontal axis);

[0027]FIG. 3 is a sample plot of voltage values of a video output recorded
from a single pixel using the CDS method as plotted over the course of 50
frames, with the peak corresponding to an incident electron hit;

[0028]FIG. 4 is a direct bombardment detector having an active pixel array
with an incident radiation beam;

[0029]FIG. 5 is a perspective view of the active pixel array of FIG. 4
with a selected pixel illustrated as enlarged with electrical components
symbolically indicated;

[0030]FIG. 6 is a symbolic of a photodiode voltage response profile of a
single pixel resulting when an incident electron hits the pixel at a
first time instance and another electron hits the pixel as a later time
instance, and a reset pulse profile (with photodiode voltage represented
along the vertical axis and time represented along the horizontal axis);

[0031]FIG. 7 a sample plot of voltage values of a video output recorded
from a single pixel as plotted over the course of 50 frames with curves
for a first sampling and a subsequent sampling; and

[0032]FIG. 8 is a sample plot of differences of voltage values
corresponding to the data of FIG. 7 of the video output recorded from a
single pixel as plotted over the course of 50 frames, with the detected
peak corresponding to an incident electron hit.

DETAILED DESCRIPTION

[0033]The detailed description set forth below in connection with the
appended drawings is intended as a description of the presently preferred
embodiment of the invention, and is not intended to represent the only
form in which the present invention may be constructed or utilized.
Reference throughout the detailed description to "one embodiment" or "an
embodiment" means that a particular feature, structure, or characteristic
described in connection with the embodiment is included in at least one
embodiment of the present invention. Thus, appearances of the phrases "in
one embodiment" or "in an embodiment" in various places throughout this
detailed description are not necessarily all referring to the same
embodiment. The following description is given by way of example, and not
limitation. Given the above disclosure, one skilled in the art could
devise variations that are within the scope and spirit of the invention
disclosed herein. Further, the various features of the embodiments
disclosed herein can be used alone, or in varying combinations with each
other and are not intended to be limited to the specific combination
described herein. Thus, the scope of the claims is not to be limited by
the illustrated embodiments. In the following description, numerous
specific details are shown to provide a thorough understanding of
embodiments of the invention. One skilled in the relevant art will
recognize, however, that the invention may be practiced without one or
more of the specific details, or with other methods, components,
materials, etc. In other instances, well-known structures, materials, or
operations are not shown or described to avoid obscuring aspects of the
invention. It is further understood that the use of relational terms such
as first and second, and the like are used solely to distinguish one from
another entity without necessarily requiring or implying any actual such
relationship or order between such entities.

[0034]Direct bombardment detectors in electron microscopy offer a number
of significant advantages over traditional means of detection and
imaging. By direct bombardment detection, primary image-forming electrons
may be detected by impinging these electrons directly on a pixelated
detector without the use of any intervening energy down-conversion
techniques. The impinging electrons deposit some of their energy in the
detector in the form of a cloud of secondary electrons. The cloud of
secondary electrons is then detected by techniques commonly used in
optical image sensors such as CCDs or CMOS image sensors. The advantages
of direct bombardment detection may include high sensitivity, for
example, detection of individual primary electron with a signal-to-noise
ratio (SNR) over 10, and improved spatial resolution. Direct bombardment
detection also offers relatively increased speed of image data
acquisition through high operation rates and large-scale integration of
read-out and digitization functions.

[0035]The advantages described above may be realized in a number of ways
in electron microscopy. For example, the very high sensitivity and high
spatial resolution are of particular benefit for determination of
structure and conformation of highly sensitive sample specimens such as
biological materials. The benefit of high sensitivity translates into
relatively lower electron dose on the sample for a given SNR in the
image. The higher spatial resolution can also be used to reduce the total
sample dose. With a direct detector, lower microscope magnification is
required to achieve the same resolution as a detector with poorer spatial
resolution. Lower magnification operating points tend to reduce dose on
the sample and confer additional advantages such as improved stability.

[0036]Direct bombardment detection may further offer advantages beyond
dose reduction for use with electron beam sensitive and other unstable
materials in electron microscopy. These advantages arise from the method
of operation of a direct bombardment detector. Unlike traditional
detectors such as CCDs and CMOS image sensors that accumulate
photo-generated charges in their pixels prior to the read-out of an image
frame, direct bombardment detectors tend to read out much faster and more
frequently. This is possible because of their high single primary
electron sensitivity which provides high readout speed without the
penalty of introducing unacceptable levels of readout noise in each
frame.

[0037]Counting with a direct bombardment detector is achieved by operating
the imager at very high frame rates thereby drastically reducing the
number of recorded primary electrons per frame. This may also be done in
concert with reducing the electron beam intensity. In this "sparse"
readout process, each primary electron produces secondary charge in a
small and distinct cluster of pixels whose signal (in analog-to-digital
converter units--ADCs) exceeds the background (unexposed) signal level
(noise level) by an amount that allows detection with high confidence
(high SNR). Cluster centers are recorded as high-energy electron
interaction events in a separate accumulation memory. By not integrating
charge, the wide statistical variation in the amount of charge produced
by individual interactions no longer degrades the SNR of the resulting
image. Similarly, by recording only the cluster center (rather than
integrating charge from a cluster of pixels) the spatial resolution of
the resulting image is improved. Key to the success of this method is the
ability to achieve sufficiently high frame rates to avoid interaction
"pile-up" while operating at practically useful beam intensities.

[0038]To achieve the fast readout speed required for a large format direct
bombardment detector to be used at convenient beam intensities in TEM
while maintaining high detection efficiency and signal to noise ratio,
several strategies are employed in the digitization and readout process.

[0039]A first strategy for fast readout is to incorporate
Analog-to-Digital Converters (ADCs) into the design of the sensor itself.
This avoids the need to bring analog signals off of the chip for
digitization in external circuits. External digitization becomes
especially cumbersome when many parallel outputs are required in order to
decrease frame time (increase readout speed). CMOS image sensor designs
are known to incorporate a digitization circuit for each row (or column)
in a 2-dimensional array. The counting mode of the direct bombardment
detector be implemented with many parallel digitization circuits
(currently one per column).

[0040]Digitization precision (number of bits used to represent output
voltage) is also optimized to maximize readout speed in counting mode.
Because the counting method does not rely on an extremely accurate
measurement of the deposited energy, the counting direct bombardment
detector can digitize the signal to fewer bits and retain the same
performance. A single bit can record the presence or absence of an event
in a pixel. More bits of precision are required, for example, to
implement centroid algorithms for achieving sub-pixel resolution.
Digitizing to lower, user settable precision in the direct bombardment
detector significantly reduces the digitization time (requiring less
conversion clocks), and allows more pixel values to be packed and
transferred off the chip within a given bandwidth limitation for data
transfer. The tradeoff between using fewer bits (higher quantization
noise) and sensitivity to events that deposit small quantities of charge
can be optimized based on sensor parameters and the energy of the
incident high-energy particle beam.

[0041]Additional strategies for increasing and optimizing the readout rate
of the counting for a direct bombardment detector involve the signal
sampling methods. As discussed above, it is well known in the art to use
Correlated Double Sampling (CDS). The method uses two sequential reads of
the sensor array, the first immediately following reset of the sensing
diodes, and the second following a predefined integration period.
Subtraction of the two values obtained for each pixel results in a signal
value free of variations in pixel offset (fixed pattern noise) and reset
noise (kTC noise).

[0042]According to aspects of the present invention, there is provided a
readout method to increase the readout speed of an imaging sensor for the
detection of high-energy electron or otherparticle hits. As discussed
above, the reset time constant for the pixel voltage can be a bottleneck
limiting the highest frame rate of the sensor. To increase the frame rate
beyond the limit of the reset time constant means that the reset will be
insufficient and the voltage of the photodiode will not return back to
the original dark level.

[0043]According to an aspect of the present invention, there is provided a
method of high-energy particle imaging by particle counting with an
active pixel direct bombardment detector, such as the active pixel direct
bombardment detector 10. Referring now to FIG. 4 there is depicted a
detector 10 having an array of pixels 12 (with individual pixels denoted
as pixels 14) with an incident radiation beam 16. The detector 10 may
take the form of any of those devices which are well known to one of
ordinary skill in the art. For example, where the detector 10 may be an
electron detector. Other detectors may be configured to sense protons,
neutrons, and other subatomic particles, x-rays and other ionizing
radiation according to those which are well know to one of ordinary skill
in the art. The detector 10 may have a design in accordance with those
teachings of U.S. Pat. No. 7,262,411 entitled "Direct Collection
Transmission Electron Microscopy" the complete contents of which are
incorporated herein by reference. As such, the detector 10 may be based
on active pixel sensors used in direct bombardment mode to achieve direct
detection of primary electrons without use of a scintillator screen. The
detector 10 may include an active pixel array comprising charge
collection diodes that collect secondary electrons generated when a
primary electron passes through the thin epitaxial silicon layer in which
the p-n junction of each diode is formed. In addition, while the array of
pixels 12 is illustrated as a 2D array, it is contemplated that the array
of pixels may be a single row type, 3D or other dimensional
configuration.

[0044]The method includes the step of providing an active pixel direct
bombardment detector 10 including the array of pixels 12. Each pixel 14
is characterized by a reset time constant. The method further includes
sampling the pixel voltage of each pixel 14 at a first time. The method
further includes applying a pixel reset voltage to each pixel 14 for a
reset time interval less than the reset time constant. The method further
includes sampling the pixel voltage of each pixel 14 at a second time.
The method further includes computing a difference between the sampled
pixel voltages at the first and second times to detect a particle
interaction event occurring between the first and second times.

[0045]Referring now to FIG. 5 there is depicted a perspective view of the
array of pixels 12 of FIG. 4 with a selected pixel 14 illustrated as
enlarged with electrical components symbolically indicated. In the
particular embodiment illustrated the pixel 14 includes a photodiode 18,
a reset switch 20 (in the form of a reset transistor 22), a pixel reset
voltage 24, a read transistor 26, a select transistor 28, and an output
30. As one of ordinary skill in the art will recognize, this arrangement
is a simple 3T (3 transistor) arrangement with the reset switch 22 being
used to connect the photodiode 18 to the reset voltage 24 and the output
30 being used to sense the pixel voltage, and more particularly the
photodiode voltage representing an accumulated charge. As one of ordinary
skill in the art will further understand, the pixel 14 of the present
invention may be implemented with other electrical components and
configurations, such as with 4T or 5T designs. The value of the reset
voltage 24 is chosen, as is well know to one of ordinary skill in the
art, so as to effectively optimize the performance of the sensor or
detector with respect to various specifications, such as dynamic range
for example, while staying within a safe margin of the maximum operating
capabilities of the device.

[0046]The detector 10 may further include a control device 32 and a
sampling device 34. The control device 32 and sampling device 34 may be
integrated into a single overall electrical component. The reset switch
22 is controlled by the control device 32. The pixel reset voltage 24 may
be applied to the pixel 14, and more specifically to the photodiode 18,
by sending a reset pulse to control the reset switch 22 connected to a
reset voltage 24 to selectively connect each pixel to the reset voltage
24. The output 30 is connected to the sampling device 24 for sensing the
pixel voltage associated with the photodiode 18. In this regard, one of
ordinary skill in the art will recognize that other electronic components
connected to the output 30 may include an amplifier and an
analog-to-digital converter.

[0047]In addition, the detector 10 may include a sensing volume 36 with a
dimension (T) parallel to an incident radiation beam less than 50
microns. Preferably, the dimension T is less than 15 microns. Such a thin
layer may be used to avoid collecting charge from primary particle
interactions from deep within the device that would tend to reduce
spatial resolution.

[0048]FIG. 6 is a symbolic of a photodiode voltage response profile of a
single pixel, such as pixel 14 resulting when an incident electron hits
the pixel at a first time instance and another electron hits the pixel as
a later time instance, and a reset pulse profile (with photodiode voltage
represented along the vertical axis and time represented along the
horizontal axis). FIG. 6 represents a sample illustration of an
implementation of the method according to an aspect of the present
invention. The top curve is the reset pulse profile. As is shown, the
reset pulse profile includes five periodic reset pulses, individually
denoted RP1, RP2, RP3, RP4 and RP5. Each of the reset pulses extends over
a reset time interval. The reset pulses repeat in equal intervals and the
reset time intervals are likewise equal. Immediately following each of
the reset pulses RP1-5 are integration time intervals, individually
denoted as I1, I2, I3, I4 and I5.

[0049]Referring now to the lower curve representing the photodiode voltage
response profile, the photodiode voltage or pixel voltage is at an
initial state of the reset voltage level, designated RL. Just subsequent
to the reset pulse RP1 at time instance 1 the pixel voltage is sampled.
This likewise occurs periodically after each of the other reset pulses
RP2-5 at time instances 2-5. It is contemplated that such sampling may
occur in close temporal proximity to the end of each reset pulse.
However, such sampling is to be effectuated consistently at whatever time
lag in comparison to each reset pulse. It is noted that sampling of the
pixel voltages occurs in the context of an overall readout process and
readout time interval. (frame time). It is contemplated that the multiple
pixels 14 that make up a multi-dimensional array of pixels 12 may be
reset and sampled sequentially, in parallel, or some combination of the
two.

[0050]Approximately at the middle portion of the integration time interval
I1, there is indicated an interaction event E1. This corresponds to a
first external signal hit or high-energy particle hitting the photodiode
18. This results in the photodiode or pixel voltage to drop down from the
initial reset voltage level RL. Subsequently, reset pulses RP2, RP3 and
RP4 expose the photodiode 18 to the pixel reset voltage 24. However, as
mentioned above, the method includes applying a pixel reset voltage to
each pixel 14 for a reset time interval less than the reset time
constant. Subsequent to the interaction event E1, the reset pulse RP2
results in the application of the pixel reset voltage 24 for a reset time
interval less than the reset time constant.

[0051]As is illustrated, after the reset pulse RP2 at the time instance 2,
the sampled pixel voltage is well below the reset voltage level RL. This
is because the reset time interval in the new method is not sufficiently
long enough to allow the pixel voltage to approach the pixel reset
voltage 24. In fact, the reset time interval is significantly shorter
than the reset time constant, which is the time interval necessary for
each pixel response to the reset voltage to reach approximately 63% of
its final asymptotic value. Even after additional reset pulses RP3 and
RP4, the pixel voltage has still just recovered to about half of the
initial voltage level RL. Thus at time instance 14, the pixel 14, and
more specifically the photodiode 18, is only partially reset. In this
regard, it is understood that in order for the pixel 14 to be fully reset
by a single reset pulse, such a reset pulse would have to have a reset
time interval much longer than that plotted in FIG. 6. It is understood
that in prior art methods, one would desire to set the reset time
interval to at least three times the reset time constant to allow the
photodiode to be fully reset within some margin of acceptability.

[0052]As mentioned above, the method of the present invention includes
sampling the pixel voltage at a first time and at a second time following
a reset, such as at time instances 1 and 2 in FIG. 6. In between these
two samplings, the interaction event E1 occurred. A significant
difference between the two sampled pixel voltages is apparent. In
contrast, the difference between subsequent sampled pixel voltages (e.g.,
the difference between sample pixel voltages at time instances 2 and 3,
and the difference between sample pixel voltages at time instances 3 and
4) are similar and in any case much less than the difference associated
with the time interval or frame between time instances 1 and 2 during
which the interaction event E1 was observed.

[0053]Approximately at the middle portion of the integration time interval
14, there is indicated an interaction event E2. This corresponds to a
second external signal hit or high-energy particle hitting the pixel 14.
This causes the photodiode or pixel voltage to drop down to a level even
lower than as was recorded at the first interaction event E1.
Subsequently, the reset pulse RP5 exposes the photodiode 18 to the pixel
reset voltage 24. The difference between pixel voltages sampled at time
instances 4 and 5 may be computed and such difference is much larger than
the differences between sampled voltages associated with time intervals
between time instances 2 and 3, and 3 and 4. This spike in difference
allows for the determination that another interaction event E2 has
occurred. This is accomplished without having to fully reset the pixel
14. Computing the difference between pixel voltages sampled in sequential
frames can be carried out in any number of ways including both analog
subtraction and subtraction of digital values. Similarly, these steps
could be effectuated entirely within detector 10, in hardware, firmware,
software or any combination of the foregoing. The computing of the
difference may be achieved through a comparison of analog or digital
signals of the pixel voltages.

[0054]The method of the present invention recognizes that the difference
between sequentially taken samples interleaved between relatively short
pixel reset pulses can be used to detect the external signal event. The
present method does not require the full reset of the pixel 14.
Therefore, the method is not limited by the reset time constant, as are
other prior art methods. Additionally, only one sample is needed after
each reset, which makes the readout process even faster than the
conventional methods that require two samples after each reset. (such as
CDS and pseudo-CDS methods)

[0055]Referring now to FIG. 7 there is depicted a sample plot of voltage
values of a video output recorded from a single pixel as plotted over the
course of 50 frames with curves for a first sampling and a subsequent
sampling., the first sampling being taken shortly after a relatively
short reset pulse and the second sampling being just before application
of the next reset pulse. Referring to FIG. 8 there is depicted a sample
plot of differences of voltage values corresponding to the first read
data of FIG. 7. This data is the video output recorded from a single
pixel using the single difference read method of the present invention as
plotted over the course of 50 frames.

[0056]FIG. 7 shows sampled pixel voltages recorded for a pixel 14 before
and after the integration period for a series of 50 frames around the
detection of a particle event in one frame when the reset pulse is
significantly shortened. The particle hit the pixel in the 22nd frame
(after the first read, but before the second read) and the charge
generated in the pixel reduced the output voltage that was sampled
(discharged the photodiode). At the start of the next frame (23rd frame),
the pixel 14 was reset with a short reset pulse that only partially
re-charged the photodiode. Subsequent resets in following frames continue
to partially recharge the photodiode until after approximately 20 frames
the diode is approaching a fully recharged state. Thus, in this example,
the reset time interval is less than 1/20th that required to fully
reset the photodiode 18.

[0057]Referring again to FIG. 7, there is depicted a sample plot of
voltage values of a video output recorded from a single pixel as plotted
over the course of 50 frames with curves for a first sampling and an
immediate subsequent sampling. From examination of FIG. 7, both the first
and second reads of a pixel 14 in frames following the interaction result
in voltages that track each other to the fully recovered state. This
suggests that it is possible to use only the first (or second) reads to
identify the occurrence of a particle interaction. Any negative spike in
output voltage followed by a recovery showing the characteristic reset
time constant is indicative of a particle interaction event. The "single
read difference method" of the present invention uses only the difference
between the sampled voltages of the first (or second) reads in sequential
frames to detect an incident particle hit. That is, the second (or first)
reads are not used and those samples are not actually taken. The data is
only shown in FIG. 7 for illustrative purposes.

[0058]To further illustrate this method, the difference values for the
first reads in the series of sequential frames shown in FIG. 7 are
presented in FIG. 8. FIG. 8 depicts a sample plot of differences of
sample voltages corresponding to the data of FIG. 7 of the video output
recorded from a single pixel as plotted over the course of 50 frames.
FIG. 8 demonstrates that it is possible to detect the incident particle
events using only the difference between the sampled voltages of first
reads in sequential frames. Due to the abbreviated pixel reset, the
detected peak value in the method is smaller as compared to that obtained
with prior art methods. The duration of each reset pulse (reset time
interval), given the time constant characteristics of the particular
detector, the pixel sensitivity, noise level, particle energy, frame
time, and other factors may be adjusted so as to be optimized for
counting. Obviously this method is not suitable for charge integration
methods, where a measure of the precise quantity of charge generated is
desired, because charge detection is not quantitative. However, because
the method only requires single reads near the start of the recovery
curve, the time between a pixel reset and the subsequent read can be
extremely short. The method is ultimately limited to particle arrival
rates that allow sufficient recovery of the reset level to make
subsequent events detectable. Furthermore, it is contemplated that the
present invention may include signal-processing facilities (i.e. FPGA) at
or very near the detector chip to carry out the detection and counting
operations disclosed here in real-time. This capability allows large
arrays (>>2K by 2K) to be used at high frame rates (>100 frames
per second) without exceeding the data handling and storage capabilities
of ordinary laboratory computers.

[0059]According to another aspect of the present invention, there is
provided a method of high-energy particle imaging by particle counting
with an active pixel direct bombardment detector, such as detector 10.
The method includes providing the active pixel direct bombardment
detector 10 including an array of pixels 12. Each pixel 14 is
characterized by a reset time constant. The method further includes
periodically applying a pixel reset voltage to each pixel 14 for a reset
time interval less than the reset time constant. The method further
includes periodically sampling the pixel voltage of each pixel 14
subsequent to each periodic application of the pixel reset voltage to
each pixel 14. The method further includes computing a difference between
each sampled pixel voltage and each immediately subsequent sampled pixel
voltage to detect a particle interaction event occurring between the
sampling of compared pixel voltages.

[0060]According to another aspect of the present invention there is
provided a direct bombardment detector 10 for high-energy particle
imaging for use with the radiation beam 16. The detector 10 includes the
array of pixels 12. Each pixel 14 is characterized by a reset time
constant. The array of pixels 12 is positionable with the radiation beam
16 being incident upon the array of pixels 12. The detector 10 further
includes the reset voltage 24. The detector 10 further includes the reset
switch 20 connected to the reset voltage 24 and each pixel 14. The reset
switch 20 is configured to selectively connect each pixel 14 to the reset
voltage 24 for a reset time interval less than the reset time constant in
response to a reset pulse. The detector 10 further includes the sampling
device 34 that is configured to sample the pixel voltage of each pixel 14
at a first time prior to connection of each pixel 14 to the reset voltage
24 and at a second time after connection of each pixel 14 to the reset
voltage 24. The detector 10 further includes the control device 32 that
is configured to compute a difference between the sampled pixel voltages
at the first and second times.